Improved cathode ray tube having a glass envelope coated with crystallized glass

ABSTRACT

A cathode ray tube including an electron gun enclosed by a glass envelope including a neck portion is disclosed in accordance with the teachings of the present invention wherein arcing between electrodes of the cathode ray tube is minimized by providing a coating of crystallized glass formed by heat-treatment of sealing glass, on the surface of the inner wall of said neck portion and coextensive with said electron gun, and a coating of conductive material, spaced from said coating of crystallized glass, on the surface of the inner wall of said neck portion. A method of treating a cathode ray tube to minimize arcing between the electrodes thereof is also disclosed.

IMPROVED CATI-IODE RAY TUBE HAVING A GLASS ENVELOPE COATED WITll-llCRYSTALLIZED GLASS Inventors: Tomoichi Kubo; Masakatsu Itazu;

I-Iirofumi Yamashita, all of Tokyo, Japan Assignee: Nippon ElectricCompany, Limited,

Minato-ku, Tokyo, Japan Filed: Jan. 20, 1971 Appl. No.: 107,900

Foreign Application Priority Data Jan. 23, 1970 Japan 45/6644 US. Cl313/64, 313/220, 313/317 References Cited UNITED STATES PATENTS 9/1956Amdursky et a1. 220/21 A 1451 Sept. 11, 1973 Primary Examiner-Rudolph V.Rolinec Assistant ExaminerWilliam H. Punter Att0rney--Marn &.langarathis [5 7] ABSTRACT A cathode ray tube including an electron gunenclosed by a glass envelope including a neck portion is disclosed inaccordance with the teachings of the present invention wherein arcingbetween electrodes of the cathode ray tube is minimized by providing acoating of crystallized glass formed by heat-treatment of sealing glass,on the surface of the inner wall of said neck portion and coextensivewith said electron gun, and a coating of conductive material, spacedfrom said coating of crystallized glass, on the surface of the innerwall of said neck portion. A method of treating a cathode ray tube tominimize arcing between the electrodes thereof is also disclosed.

2 (Ilaims, 1 Drawing Figure PATENTED 1 I973 3.758.802

INVENTORS Tomoichi Kubo Masukotsu ltozu Hirofumi Yomoshito ATTORNEYSIMPROVED CATl-EODE RAYTUBE HAVllNG A GLASS'ENVELOPE COATED WITH.CRYSTALLIZED GLASS This invention relates to cathode ray tubes and moreparticularly to cathode ray tubes wherein arcing between electrodesthereof is minimized.

The cathode ray tube adapted for'television and especially colortelevision reception requires high voltage for the effectiveoperation'thereof, which voltage attains a value up to 27,000 volts.Such cathode ray tubes usually consist of a glass envelope having a neckportion and a viewing screen disposed at opposite ends of said envelope.Within the neck portion is an electron gun structure for generating andfocusing a beam of electrons upon the viewing screen. This electron gunstructure includes a cathode which serves as the source for the beam ofelectrons and a plurality of electrodes. One or several of theseelectrodes may comprise accelerating anodes and others of saidelectrodes may comprise control grids which cooperate with theaccelerating anodes to focus the beam of electrons onto the viewingscreen with the proper intensity. In addition, the surface of the innerwall of the neck portion of the glass envelope may be coated withconductive material such as graphite which acts as an additional anodeto provide a final focus to the beam of electrons.v

During operation of the cathode ray tube, the application of theaforementioned high voltage to the electrodes thereof often results in avacuum discharge phenomenon commonly known as arcing, which may damagethe display screen, the glass envelope and portions of the electron gun.Arcing may be attributed to roughness of, projections from, andirregularities in the electrodes of the cathode ray tube. Accordingly,during the manufacture of cathode ray tubes it is preferred to employ aprocess commonly known as high voltage treatment" or spot knockingwhereina voltage greater than the normal operating voltage of thecathode ray tube is applied to the electrodes thereof. The appliedvoltage is usually equal to twice the normal operating voltage or about60,000 volts. During such high voltage treatment arcing occurs to induceenough current to flow between the electrodes such that thecontaminations and irregularities thereof are fused or melted off,thereby minimizing the probability of arcing duringnormal operation ofthe cathode ray'tube.

An attendant disadvantage of the high voltage treatment as heretoforeutilized by the prior art is the production of a potential gradientalong the surface of the inner wall of the neck portion of the glassenvelope due to the application of the high voltage between the cathodeand the graphite coating on the glass envelope. Since the graphitecoating is supplied with the high anode voltage, a portion of thesurface of the inner wall of the neck portion of the glass envelope ismaintained at a relatively high voltage potential. Consequently theglass wall undergoes a concentrated electron bombardment from anelectrode supplied with a relatively low voltage potential. Theconcentrated electron bombardment produces a heating of the glass wallresulting in punctured and cracked portions of the glass envelope.Accordingly, the magnitude of the voltages applied to the electrodes ofthe cathode ray tube during thehigh voltage treatment and the timeduration of such high voltage treatment must be strictly limited toavoid damage of the glass envelope. Unfortunately, such limitations mayforestall the successful completion of the high voltage treatment suchthat the aforementioned irregularities which contribute to arcing arenot removed. I

Therefore, it is an object of the present invention to provide animproved cathode ray tube that is particularly adapted to withstandcomplete high voltage treatment.

It is another object of this invention to provide a cathode ray tubewherein arcing between the electrodes thereof is substantially minimizedduring normal operation.

A further object of the present invention is to provide a cathode raytube which is adapted to receive a higher operating voltage thenheretofore utilized by the prior art.

Yet another object of this invention is to provide a method of treatinga cathode ray tube whereby arcing between the electrodes thereof isminimized during normal operation. of a Various other objects andadvantages of the invention will become clear from the followingdetailed description of an exemplary embodiment thereof and the novelfeatures will be particularly pointed out in connection with theappended claims.

In accordance with this invention an improved cathode ray tube includingan electron gun enclosed by a glass envelope is provided wherein a filmof crystallized glass formed by heat-treatment of sealing glass, iscoated on the surface of the inner wall of said glass envelope, thecoating being coextensive with said electron gun. A cathode ray tubetreated in accordance with the method of the present invention exhibitsminimal arcing between the electrodes thereof during normal operation.

The invention will be more clearly understood by reference to thefollowing detailed description of an exemplary embodiment thereof inconjunction with the accompanying drawing which is a cross sectionalview of a portion of a cathode ray tube in accordance with the presentinvention.

Referring now to the drawing, there is illustrated a portion of acathode ray tube which may be utilized in a color television receiverand includes a unipoten'tial electron gun I comprised-of a cathodeassembly 2, a

first electrode 3, a'second electrode 4, a third electrode 5, a fourthelectrode 6 and a fifth electrode 7. The electron gun assembly 1 issealingly enclosed in the neck portion 8 of an evacuated glass envelope.It is understood that a display screen, although not shown, is providedat one end of the glass envelope oppositely disposed from stem portion11 of the neck 8. The cathode assembly 2 serves as the source for thebeam of electrons and may comprise a conventional cathode such as anindirectly heated oxide coated cathode. The various electrodes 4 through7 may comprise control grids, accelerating anodes and focusingelectrodes, well known in the prior art, the-description of which neednot be set forth in detail herein for a complete understanding of thepresent'invention. A portion of the surface of the inner wall of neck 8is coated with conductive material 9 such as a graphite layer, whichcoating may extend from the mid-portion of neck 8 into the bulb portionof the glass envelope, not shown. It is recognized by those skilled inthe cathode ray tube art that conductive coating 9 may act as anaccelerating anode or a focusing electrode.

During the manufacture of'the cathode ray tube one or more of theelectrodes 3 through 7 as well as the conductive coating 9 may beprovided with irregularities such as contaminations, roughness orprotrusions, which may result in deleterious arcing effects duringnormal operation of the cathode ray tube. Accordingly, it is preferableto provide high voltage treatment of the assembled cathode ray tube toremove such irregularities from the electrodes. In the high voltagetreatment of cathode ray tube as illustrated in the drawing, a highvoltage of about twice the magnitude of the normal operating voltage isapplied to electrodes 5 and 7-and a low potential such as groundpotential is applied to electrodes 3, 4 and 6. The high voltages appliedto the aforementioned electrodes results in arcing and electrons will beemitted from electrode 6 to bombard the electrodes 5 and 7- oppositelydisposed thereto. it is clear however that the electrons emitted fromelectrode 6 will also bombard that portion of the surface of the innerwall of neck portion 8 that is oppositely disposed from electrode 6. Theconcentrated bombardment of the glasswall of neck 8 may produce aheating of the glass wall causing thermal breakdown thereof resulting inelectrical-puncture or breakage of theglass.

According to the present invention, this portion of the glass tube wallsurface is provided with the abovementioned kind of protective coatingfilm 10 made of materials different from that of the glass tube so as toencircle the electrodes. Then, the electrons would be reflected orabsorbed by this protective coating film, so that the direct effect ofthe electrons does not reach up to the glass wall. If the protectivecoating 10 admits of a rough surface then the increase in surface areais effective to decrease the concentration of electrons per unit areacommunicated thereto, resulting in a further improvement of theprotective qualities of the coating The protective coating 10 ispreferably comprised of a film of material such as sealing glassmanufactured by the Corning Glass Works. A complete description of themethod of making sealing glass as well as the composition and attributesthereof may be found in U.S.

Pat. No. 2,889,952 which issued to SA. Claypoole on June 9, 1959. Itwill suffice therefore, for a complete understanding of the presentinvention to note that the protective coating 10 is comprised of a filmof crystallized glass formed by heat-treatment of sealing glass.

The manner in which the protective coating 10, having a rough surface,may beattained will now be described. 200 grams of crystalline glasspowder such as sealing glass powder, having a particle size of less than100 mesh is suspended in a mixture of 125 c.c. of acetone and 0.5 gramsof nitrocellulose. The suspension is then deposited on the surface ofthe inner wall of neck portion 8 such as by spraying or by applying thesuspension with a brush. It should be noted that the thickness of theprotective coating 10, although critical, admits Consequently thethickness of the protective coating 10 is preferably within the range ofto 800 microns. The suspension, after being deposited on the surface ofthe inner wall of the neck portion 8, is subjected to natural drying andis then heated in air at a temperature of about 450C to rigidly securethe protective coating to the glass wall.

A cathode ray tube provided with a film of protective coating 10 asdescribed hereinabove is particularly adapted for high voltage treatmentto thereby minimize arcing between electrodes thereof during normaloperation. it has been found that the high voltage potential applied tothe electrode of a cathode ray tube of the present invention during highvoltage-treatment thereof may exceed the voltage potentials hereinbeforeapplied by the prior art by approximately 10,000 to 20,000

volts. Experimentation has proven that the damage to cathode ray tubesprepared in accordance with the present invention during high voltagetreatment thereof has been substantially reducedeven when voltages inthe'range of 60,000 to 80,000 volts have been applied. The followingtable provides an impressive indication v of the improved resultsachieved by the present inven- Rate of damage of tubes caused by thehigh voltage treatment Voltage used in the high voltage treatment Whilethe present invention has been particularly described with reference toonly a single electron gun included in a cathode ray tube it is readilyapparent that a plurality of electron guns may be employed in accordancewith this invention and the cathode ray tube may comprise a conventionalthree gun cathode ray tube conventionally utilized in the colortelevision art. In addition the particular application of the cathoderay tube is not limited to television and may include'other uses such asan oscilloscope, a'radar screen, an optical read out device or the like.Therefore, it will be obvious to those skilled in the art that theforegoing and various other changes and modifications in form anddetails may be made without departing from the spirit and scope of theinvention. It is intended that the appended claims be interpreted asincluding all such changes and modifications.

What is claimed is:

1. In a cathode ray tube including an electron gun en closed'by' a glassenvelope, the improvement comprisofa wide range. If the thickness of thecoating 10 is too thin then proper protection is not afforded the glasswall of neck portion 8 and electron bombardment during high voltagetreatment may produce sufficient heat to damage the wall. Conversely, ifthe thickness of the coating 10 is too thick, the coating will readilypeel off.

ing a film consisting of crystallized glass formed by heat-treatmentsealing glass coated on the surface of the inner wall of said glassenvelope and coextensive with said electron gun.

2. The improvement of claim 1 wherein said film consisting ofcrystallized glass includes a rough surface.

I t t i

1. In a cathode ray tube including an electron gun enclosed by a glassenvelope, the improvement comprising a film consisting of crystallizedglass formed by heat-treatment sealing glass coated on the surface ofthe inner wall of said glass envelope and coextensive with said electrongun.
 2. The improvement of claim 1 wherein said film consisting ofcrystallized glass includes a rough surface.